JAKs are kinases which phosphorylate a group of proteins called Signal Transduction and Activators of Transcription or STATs. When phosphorylated, STATs dimerize, translocate to the nucleus and activate expression of genes which lead to, amongst other things, cellular proliferation.
The central role played by the JAK family of protein tyrosine kinases in the cytokine dependent regulation of both proliferation and end function of several important cell types indicates that agents capable of inhibiting the JAK kinases are useful in the prevention and chemotherapeutic treatment of disease states dependent on these enzymes. Potent and specific inhibitors of each of the currently known four JAK family members will provide a means of inhibiting the action of the cytokines that drive immunological and inflammatory diseases.
Myeloproliferative disorders (MPD) include, among others, polycythemia vera (PV), primary myelofibrosis, thrombocythemia, essential thrombocythemia (ET), idiopathic myelofibrosis (IMF), chronic myelogenous leukemia (CML), systemic mastocystosis (SM), chronic neutrophilic leukemia (CNL), myelodisplastic syndrome (MDS) and systemic mast cell disease (SMCD). JAK2 is a member of the JAK family of kinases in which a specific mutation (JAK2V617F) has been found in 99% of polycythemia vera (PV) patients and 50% of essential thrombocytopenia (ET) and idiopathic myelofibrosis (MF). This mutation is thought to activate JAK2, giving weight to the proposition that a JAK2 inhibitor will be useful in treating these types of diseases.
Asthma is a complex disorder characterized by local and systemic allergic inflammation and reversible airway obstruction. Asthma symptoms, especially shortness of breath, are a consequence to airway obstruction, and death is almost invariably due to asphyxiation. Airway Hyper Responsiveness (AHR), and mucus hyper secretion by goblet cells are two of the principle causes of airway obstruction in asthma patients. Intriguingly recent work in animal experimental models of asthma has underscored the importance of IL-13 as a key player in the pathology of asthma. Using a specific IL-13 blocker, it has been demonstrated that IL-13 acts independently of IL-4 and may be capable of inducing the entire allergic asthma phenotype, without the induction of IgE (i.e. in a non-atopic fashion). This and other models have pointed to an important second tier mechanism for elicitating the pathophysiology of asthma, that is not dependent on the production of IgE by resident B-cells or the presence of eonisophils. A direct induction of AHR by IL-13, represents an important process that is likely to be an excellent target for intervention by new therapies. A contemplated effect of a JAK2 inhibitor to the lungs would result in the suppression of the local release of IL-13 mediated IgE production, and therefore reduction in histaminine release by mast cells and eosinophils. This and other consequences of the absence of IL-13 indicate that many of the effects of asthma may be alleviated through administration of a JAK2 inhibitor to the lungs.
Chronic Obstructive Pulmonary Disease (COPD) is a term which refers to a large group of lung diseases which can interfere with normal breathing. Current clinical guidelines define COPD as a disease state characterized by airflow limitation which is not fully reversible. The airflow limitation is usually both progressive and associated with an abnormal inflammatory response of the lungs to noxious particles and gases, particularly cigarette smoke and pollution. Several studies have pointed to an association between increased production of IL-13 and COPD, lending support to the proposition that the potential alleviation of asthma symptoms by use of a JAK2 inhibitor, may also be achieved in COPD. COPD patients have a variety of symptoms including cough, shortness of breath, and excessive production of sputum. COPD includes several clinical respiratory syndromes including chronic bronchitis and emphysema.
Chronic bronchitis is a long standing inflammation of the bronchi which causes increased production of mucus and other changes. The patient's symptoms are cough and expectoration of sputum. Chronic bronchitis can lead to more frequent and severe respiratory infections, narrowing and plugging of the bronchi, difficult breathing and disability.
Emphysema is a chronic lung disease which affects the alveoli and/or the ends of the smallest bronchi. The lung loses its elasticity and therefore these areas of the lungs become enlarged. These enlarged areas trap stale air and do not effectively exchange it with fresh air. This results in difficult breathing and may result in insufficient oxygen being delivered to the blood. The predominant symptom in patients with emphysema is shortness of breath.
Additionally, there is evidence of STAT activation in malignant tumors, among them lung, breast, colon, ovarian, prostate and liver cancer, as well as Hodgkins lymphoma, multiple myeloma and hepatocellular carcinoma. Chromosomal translocations involving JAK2 fusions to Tel, Bcr and PCM1 have been described in a number of hematopoietic malignancies including chronic myelogenous leukemia (CML), acute myelogenous leukemia (AML), chronic eosinophilic leukemia (CEL), myelodisplastic syndrome (MDS), myeloproliferative disease (MPD) and acute lymphocytic leukemia (ALL). This suggests treatment of hyperproliferative disorders such as cancers including multiple myeloma; prostate, breast and lung cancer; Hodgkin's Lymphoma; CML; AML; CEL; MDS; ALL; B-cell Chronic Lymphocytic Leukemia; metastatic melanoma; glioma; and hepatoma, by JAK inhibitors is indicated.
Potent inhibitors of JAK2, in addition to the above, will also be useful in vascular disease such as hypertension, hypertrophy, cardiac ischemia, heart failure (including systolic heart failure and diastolic heart failure), migraine and related cerebrovascular disorders, stroke, Raynaud's phenomenon, POEMS syndrome, Prinzmetal's angina, vasculitides, such as Takayasu's arteritis and Wegener's granulomatosis, peripheral arterial disease, heart disease and pulmonary arterial hypertension.
Pulmonary arterial hypertension (PAH) is a pulmonary vascular disease affecting the pulmonary arterioles resulting in an elevation in pulmonary artery pressure and pulmonary vascular resistance but with normal or only mildly elevated left-sided filling pressures. PAH is caused by a constellation of diseases that affect the pulmonary vasculature. PAH can be caused by or associated with collagen vascular disorders such as systemic sclerosis (scleroderma), uncorrected congenital heart disease, liver disease, portal hypertension, HIV infection, Hepatitis C, certain toxins, splenectomy, hereditary hemorrhagic teleangiectasia, and primary genetic abnormalities. In particular, a mutation in the bone morphogenetic protein type 2 receptor (a TGF-b receptor) has been identified as a cause of familial primary pulmonary hypertension (PPH). It is estimated that 6% of cases of PPH are familial, and that the rest are “sporadic.” The incidence of PPH is estimated to be approximately 1 case per 1 million population. Secondary causes of PAH have a much higher incidence. The pathologic signature of PAH is the plexiform lesion of the lung which consists of obliterative endothelial cell proliferation and vascular smooth muscle cell hypertrophy in small precapillary pulmonary arterioles. PAH is a progressive disease associated with a high mortality. Patients with PAH may develop right ventricular (RV) failure. The extent of RV failure predicts outcome. The JAK/STAT pathway has recently been implicated in the pathophysiology of PAH. JAKs are kinases which phosphorylate a group of proteins called Signal Transduction and Activators of Transcription or STATs. When phosphorylated, STATs dimerize, translocate to the nucleus and activate expression of genes which lead to proliferation of endothelial cells and smooth muscle cells, and cause hypertrophy of cardiac myocytes. There are three different isoforms of JAK: JAK1, JAK2, and JAK3. Another protein with high homology to JAKs is designated Tyk2. An emerging body of data has shown that the phosphorylation of STAT3, a substrate for JAK2, is increased in animal models of PAH. In the rat monocrotaline model, there was increased phosphorylation of the promitogenic transcription factor STAT3. In this same study pulmonary arterial endothelial cells (PAECs) treated with monocrotaline developed hyperactivation of STAT3. A promitogenic agent or protein is an agent or protein that induces or contributes to the induction of cellular proliferation. Therefore, one effect of JAK2 inhibition would be to decrease proliferation of endothelial cells or other cells, such as smooth muscle cells. A contemplated effect of a JAK2 inhibitor would be to decrease the proliferation of endothelial cells or other cells which obstruct the pulmonary arteriolar lumen. By decreasing the obstructive proliferation of cells, a JAK2 inhibitor could be an effective treatment of PAH.
Additionally the use of JAK kinase inhibitors for the treatment of viral diseases and metabolic diseases is indicated.
Although the other members of the JAK family are expressed by essentially all tissues, JAK3 expression appears to be limited to hematopoetic cells. This is consistent with its essential role in signalling through the receptors for IL-2, IL4, IL-7, IL-9 and IL-15 by non-covalent association of JAK3 with the gamma chain common to these multichain receptors. Males with X-linked severe combined immunodeficiency (XSCID) have defects in the common cytokine receptor gamma chain (gamma c) gene that encodes a shared, essential component of the receptors of interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15. An XSCID syndrome in which patients with either mutated or severely reduced levels of JAK3 protein has been identified, suggesting that immunosuppression should result from blocking signalling through the JAK3 pathway. Gene Knock out studies in mice have suggested that JAK3 not only plays a critical role in B and T lymphocyte maturation, but that JAK3 is constitutively required to maintain T cell function. Taken together with the biochemical evidence for the involvement of JAK3 in signalling events downstream of the IL-2 and IL-4 receptor, these human and mouse mutation studies suggest that modulation of immune activity through the inhibition of JAK3 could prove useful in the treatment of T-cell and B-cell proliferative disorders such as transplant rejection and autoimmune diseases. Conversely undesired inhibition of JAK3 could have a devastating effect on the immune status of an individual treated with drug.
Although the inhibition of various types of protein kinases, targeting a range of disease states, is clearly beneficial, it has been to date demonstrated that the identification of a compound which is selective for a protein kinase of interest, and has good “drug like” properties such as high oral bioavailability, is a challenging goal. In addition, it is well established that the predictability of inhibition, or selectivity, in the development of kinase inhibitors is quite low, regardless of the level sequence similarity between the enzymes being targeted.
The challenges in developing therapeutically appropriate JAK2 inhibitors for use in treatment kinase associated diseases such as immunological and inflammatory diseases including organ transplants; hyperproliferative diseases including cancer and myeloproliferative diseases; viral diseases; metabolic diseases; and vascular diseases include designing a compound with appropriate specificity which also has good drug-likeliness.
There is therefore a continuing need to design and/or identify compounds which specifically inhibit the JAK family of kinases, and particularly compounds which may preferentially inhibit one of the JAK kinases relative to the other JAK kinases, particularly JAK2. There is a need for such compounds for the treatment of a range of diseases.